Primary sclerosing cholangitis (PSC) is a fibro-obliterative cholangiopathy that causes significant morbidity and mortality and is essentially untreatable without liver transplantation. Progression toward end stage PSC is characterized by an exaggerated fibrogenic response to chronic injury, culminating in peri-portal deposition of matrix molecules that progresses to biliary cirrhosis. Deposition of cholangiocyte-derived FN forms a provisional scaffold that subsequently activates hepatic stellate cells (HSC) and thus is an early, important, and potentially reversible step in the progression of biliary fibrosis. Enhancer of zeste homologue 2 (EZH2) is a key epigenetic regulator that enzymatically mediates the tri-methylation of lysine 27 on histone 3 (H3K27me3) to silence transcription. Our novel line of investigation proposes that, in normal cholangiocytes, FN transcription is homeostatically silenced by EZH2. We also propose that, in PSC, the p300 acetyltransferase replaces H3K27 methylation with H3K27 acetylation (H3K27ac), to initiate FN transcription. Our preliminary results demonstrate that PSC liver tissue shows loss of H3K27me3, accumulation of H3K27ac, and peri-portal deposition of FN. Furthermore, p300 forms specific protein complexes that are selectively recruited to the FN promoter. Animals genetically lacking EZH2 develop biliary fibrosis and have an exaggerated response to the choline-deficient, ethanolamine- supplemented (CDE) diet with worsened biliary fibrosis. Based on this preliminary data, we propose the central hypothesis that a switch from H3K27me3 (silencing) to its opposing mark, H3K27ac (activating), epigenetically initiates FN transcription in cholangiocytes and leads to HSC activation. In Aim I, we will test the subhypothesis that FN transcription in cholangiocytes and subsequent HSC activation are regulated through a balance of H3K27 methylation via EZH2 and H3K27 acetylation via p300. In Aim II, we will test the subhypothesis that reciprocal regulation at H3K27 in cholangiocytes regulates FN deposition, HSC activation, and biliary fibrosis in vivo. The significance of these studies lies in the novel concept that epigenetic regulators allow cholangiocytes to transduce specific membrane signals into modifications of their matrix microenvironment by re-writing the histone code and generating specific fibrogenic molecules, such as FN. In turn, interventions targeting these newly discovered pathways with epigenetic pharmacology may have the capability to prevent or reverse early matrix events that drive biliary fibrosis in PSC.